Abstract The objective of this proposal is to demonstrate the feasibility of a novel pixel structure to realize low-cost avalanche gain and ultrafast radiation sensing for time-of- ight (TOF) PET. The proposed detector, which we call selenium solid-state photomultiplier (Se-SSPM) employs three major components: a substrate which houses the readout electronic circuit (CMOS readout with coincidence timing) identical to existing products; an integrated multi- well structure with fully encapsulated grids built on top of each pixel; and deposition of a thin indirect conversion a-Se photoconductive layer with thickness ranging from 10 to 35 m, also identical to existing products. The key innovations are the following: 1) unipolar time-di erential (UTD) charge sensing inside avalanche a-Se detector: UTD charge sensing within the a-Se bulk using a high-density multi-well structure over the pixel electrode of the readout circuit; 2) picosecond time-resolution of the proposed Se-SSPM to enable optimal TOF-PET. Our objective will be achieved through two speci c aims: 1) Design and fabricate prototype Se-SSPM pixel detectors; 2) Characterize the performance of prototype Se-SSPM and demonstrate UTD charge sensing and picosecond time resolution. Successful accom- plishment of these speci c aims will prove the feasibility of a low-cost indirect conversion a-Se detector structure for TOF applications, and pave the way for future development of prototype detectors. These detectors will be investigated for TOF-PET and TOF-PET/MR to achieve optimal clinical and cost e ectiveness. In addition, these detectors will be studied for the construction of a whole-body TOF-PET to maximize geometric detection eciency and reach the fundamental sensitivity limits of PET. 1